Hydraulic fluid which circulates in a hydraulic circuit of a construction machine is likely to have heat when a pressure loss or the like occurs due to flow resistance at a hydraulic device. For this reason, conventionally, the construction machine is provided with an oil cooler for cooling the hydraulic fluid.
On the other hand, when a construction machine is located under a low outside temperature condition such as in a cold district, the temperature of hydraulic fluid is liable to drop when the construction machine is in a stopped state, so that the cold hydraulic fluid is liable to negatively affect the operation of a hydraulic device when the construction machine is activated.
Here, it is considered to heat the hydraulic fluid at the time of activation of the construction machine. However, a long period of time would be required to heat the hydraulic fluid in the case that the hydraulic fluid is circulated in a hydraulic circuit having an oil cooler.
In view of this problem, when a construction machine is activated, hydraulic fluid is caused to circulate in a hydraulic circuit without passing through an oil cooler, whereby the hydraulic fluid is heated to warm up the construction machine (for example, Japanese Patent Application No. 2005-155698, which will be hereinafter referred to as Patent Literature 1).
A hydraulic circuit disclosed in Patent Literature 1 includes a hydraulic pump, a hydraulic actuator to be operated by hydraulic fluid discharged from the hydraulic pump, a control valve for controlling the operation of the hydraulic actuator, and a switch valve for switching an oil line designated as a destination of supply of hydraulic fluid, between a cooling oil line including an oil cooler and a non-cooling oil line running away from the oil cooler.
The control valve is shifted between a permitting position to permit supply of hydraulic fluid to the hydraulic actuator and a restricting position (neutral position) to restrict the supply of hydraulic fluid to the hydraulic actuator. Hydraulic fluid discharged from the hydraulic pump is led to the switch valve via the control valve.
The switch valve is shifted to a position to lead hydraulic fluid to a cooling oil line when the control valve is shifted to the permitting position, and shifted to another position to lead hydraulic fluid to the non-cooling oil line when the control valve is shifted to the restricting position.
This allows hydraulic fluid to circulate in the hydraulic circuit without passing through the oil cooler to thereby warm up the construction machine when the control valve is at the restricting position (at the time of activation of the construction machine when the hydraulic actuator is not operating).
However, the hydraulic circuit disclosed in Patent Literature 1 includes the switch valve for switching the oil line designated as a destination of supply of hydraulic fluid, in addition to the control valve for controlling the operation of the hydraulic actuator, which results in a complicated configuration of and an increase in the cost for, the hydraulic circuit.